1. Field of the Invention
The present invention relates to a process for producing a grain-oriented electrical steel sheet, the crystal grains thereof having a {110}&lt;001&gt; orientation, which is easily magnetizable in the rolling direction.
The grain-oriented electrical steel sheet is used as soft magnetic material for a core of a transformer and other electric machineries and apparatuses. Because of the recent shortage in electric power and the need to conserve energy resources, demands for grain-oriented electrical steel sheets having a low watt loss have been increasing.
2. Description of the Prior Art
Japanese Unexamined Patent Publication No. 48-69720 discloses a process for producing a grain-oriented electrical steel sheet by utilizing as the precipitation dispersion phases mainly MnS and proposes, in order to disperse MnS uniformly and at a high distribution density and hence to improve the magnetic properties of the final product, that, during the hot-rolling stage, holding be carried out at a temperature not greater than 1200.degree. C. and not less than 950.degree. C. for a period of from 30 seconds to 200 seconds.
The present inventors made repeated studies in an attempt to improve the magnetic properties of a grain-oriented electrical steel sheet, with MnS as the precipitation dispersion phases, and proposed, in Japanese Unexamined Patent Publication No. 58-42727, that Cu.sub.2 S be used, in addition to MnS, as the precipitation dispersion phases to strengthen the precipitation dispersion. In this case, the secondary cold-rolling provides a high ratio of from 50% to 80%, thereby enhancing the magnetic flux density and improving the watt loss due to refinement of the macro-grains of the final product. When an attempt is made to further decrease the watt loss, the magnetic flux density must be stabilized and the macro-grains of the final product must be refined. In order to attain this, the contents of Cu, Mn, and S must be increased, and the final cold-rolling must be carried out at a higher ratio so as to further strengthen the precipitation dispersion. However, when the Cu content is increased, cracks form in the slabs during heating due to hot-embrittlement, or the slabs may be ruptured during hot-rolling, with the result that the recovery and the operation efficiency are seriously lessened. On the other hand, when the Mn and S contents are increased, the solution temperature is enhanced, and, hence, the slabs must be thoroughly heated at a high temperature, with the result that the same problems as in the case of increasing the Cu content result.
Methods for producing a grain-oriented electrical steel sheet by utilizing Al and N as precipitation-dispersion phases are well known. However, if the precipitation-dispersion phases comprised of AlN are utilized for the so-called two-stage cold-rolling process including two cold-rollings and two annealings, to which the present invention belongs, the diameter of the macro-grains is large in the final product, and, hence, the watt loss is deteriorated.
As is well known, suppression of the growth of primary crystal grains is important for stabilizing secondary recrystallization so as to generate crystal grains having a {110}&lt;001&gt; orientation, especially when the sheet thickness of the product is thin.